1. Electrical Wire Insulation
The deterioration and breakdown of electrical insulation under stress of applied voltage has been studied, and some aspects of insulation failure are well understood. For example, it is well known that application of very high voltages (e.g., 2000-3000 volts or more) to electrical apparatus of certain configurations gives rise to the phenomenon of corona discharge and that such corona can cause progressive deterioration and failure of insulation. The corona discharge can occur under influence of both direct current (DC) and alternating current (AC) potential and can occur in both wet and dry environments. It is further well known that insulation failure due to corona discharge can be greatly reduced or eliminated by proper design of apparatus and by selection of insulation recognized as being resistant to corona attack.
However, there have been reports of unexpected failures in certain wire insulations even though the voltage to which the insulated wire was subjected was too low for corona to occur. Thus, in testing insulated wires for use in wet locations, it has been observed that the application of AC potentials in the range of only 600 volts can cause insulation failure within a few days. This unusual phenomenon has been observed for a number of semi-crystalline polymer insulation materials such as high density polyethylene (C. A. Liddicoat and B. F. Brown, Wire and Wire Products, 38, 1874 (1963) and W. D. Paist, Wire and Wire Products, 39, 1587 (1964)) and polypropylene (M. Okada, Polymer Letters 3, 407 (1965)). Tests have also shown that wire insulations of the fluorocarbon resins, polytetrafluoroethylene(PTFE), fluorinated-ethylenepropylene (FEP), and perfluoroalkoxy (PEA) fluorocarbon, are subject to failure at the same low voltage AC under wet conditions.
These relatively low voltage deteriorations of insulating ability have been called "AC wet service failure" to distinguish from the type of deterioration due to corona discharge. The "AC wet service failure" deterioration is measured by an "Insulation Resistance Test" described below.
2. The Use of Fluorocarbon Resin as Electrical Insulation.
Despite deterioration at low voltages under wet conditions, fluorocarbon resins such as tetrafluoroethylene homopolymer and copolymers are recognized as having excellent electrical properties which make them useful as electrical insulations. The commercial processing of these resins into suitable form for use as insulation for electrical wire is carried out by one of two distinctly different fabrication technologies: namely,
a. Paste Extrusion And Sintering PA1 b. Melt Extrusion PA1 (a) a melt-processible copolymer consisting essentially of units of tetrafluoroethylene and at least one comonomer represented by the formula ##STR1## wherein R.sub.1 is F, H or Cl; and when R.sub.1 is F, H or Cl, R.sub.2 can be --R.sub.F, --OR.sub.F, --R.sub.F 'X or --OR.sub.F 'X in which R.sub.F is a linear perfluoroalkyl radical of 1-5 carbon atoms, --R.sub.F ' is a linear perfluoroalkylene diradical of 1-5 carbon atoms in which the valences are at each end of the linear chain, and X is H or Cl; PA1 and when R.sub.1 is F, R.sub.2 can be ##STR2## wherein n is 1 or 2 and Y is perfluoroalkyl of 1-9 carbon atoms, or can be ##STR3## PA1 with the proviso that R.sub.1 and R.sub.2 taken together can be the diradical ##STR4## where R.sub.3 and R.sub.4 are independently --CF.sub.3 or CF.sub.2 Cl, and (b) an organo-polysiloxane that is substantially stable and nonvolatile at the temperature of melt-processing for the copolymer used and is substantially incompatible with the copolymer, said organo-polysiloxane being present in the composition in an amount of between about 0.2 and 5% by weight based on weight of the composition and being dispersed in the copolymer. PA1 (1) coating a melt-processible copolymer resin defined as above with between about 0.2 and 5% by weight based on weight of the composition of an organo-polysiloxane, PA1 (2) melting the coated resin, and PA1 (3) subjecting the melted coated resin to a shear force sufficient to disperse the organo-polysiloxane within the copolymer. PA1 (1) subjecting a melt-processible copolymer resin, defined as above, to pressure sufficient to cause the resin to pass through the zones in which the remainder of steps of the process occur, PA1 (2) melting said resin either prior to or subsequently to or simultaneously with said pressuring, and after carrying out steps (1) and (2), PA1 (3) adding to the melted resin between about 0.2 and 5% by weight based on weight of composition of an organo-polysiloxane, and PA1 (4) subjecting the mixture of step (3) to a shear force sufficient to disperse the organopolysiloxane within the copolymer.
The first of these two fluorocarbon resin fabrication technologies used for the production of wire insulation is called "paste extrusion and sintering". The fabrication of wire insulation by the "paste extrusion and sintering" technology involves, first, forming or shaping the resin mass by a specific low temperature fibrillation process known as "paste extrusion" and subsequently "sintering" the resin mass at a temperature above 327.degree. C.
The "paste extrusion and sintering" type of wire insulating fabrication is carried out with non-melt-processible tetrafluoroethylene polymers. These are polymers that cannot be melt-processed with usual melt-processing equipment because of their extremely high viscosities. Such polymers include the homopolymers of tetrafluoroethylene and copolymers of it and small amounts of comonomer, which amounts are too small to impart melt-processibility to the polymer. These polymers are prepared by the coagulation of aqueous dispersions of dispersion polymerized monomers, such as those described in Cardinal et al. U.S. Pat. No. 3,142,665.
In the "paste extrusion and sintering" process, the resin can be used to form a coating around an electrical conductor directly, or it can be used to form an unsintered tape, which can subsequently be wrapped around the conductor and sintered to form the insulated wire. Regardless of whether the insulation is formed directly or by way of unsintered tape, it is essential that the resin be fibrillatible in order that it can be fibrillated and subsequently sintered in thicknesses of about 0.1 mm or more without application of pressure and without formation of cracks in the coatings. The use of the sintering technology for commercial wire coating thus inherently requires selection of a resin composition capable of undergoing this specific fibrillation phenomenon. The non-melt-processible tetrafluoroethylene resins described above undergo the phenomenon. The unique crystal structure necessary for fibrillation in paste extrusion arises in the polymerization and is adversely affected by melting. Any tetrafluoroethylene polymer of copolymer previously heated above its crystalline melting point is not suitable for fabrication by paste extrusion and sintering.
It should be mentioned that "sintering" is a term having a specific meaning in the field of fluorocarbon resin processing and concerns the no-flow phenomenon characteristic of the sintering used in the fields of ceramics and powder metallurgy. Thus, although fluorocarbon resin sintering entails heating above the crystalline melting point, as measured by differential thermal analysis, the tetrafluoroethylene polymers applicable to this fabrication technology have such high molecular weights that they are practically form-stable, i.e., non-flowing, at usual sintering temperatures in the range of 327.degree. to about 400.degree. C. Therefore, sintering of non-melt processible resins is not the same as melt-processing (i.e., melt extrusion) of melt-processible resins. The sinterable, i.e., non-melt processible, resins do not sag or drip off wire during transit in making wire coating through the hot zone of the sintering over because their melt viscosities, measurable only by tensile creep, are in the range of 10.sup.10 to 10.sup.12 poise. This range is too "viscous" for processing by conventional melt fabrication methods. The term "sintering" is not used in connection with melt-processible fluorocarbon resins.
The second type of fabrication technique used for the production of fluorocarbon resin wire insulations is the technique of melt extrusion. In this technique conventional melt processing procedures used for other thermoplastic polymers are employed. However, only certain tetrafluoroethylene resins can be melt-processed. For example, M. I. Bro and B. W. Sandt, U.S. Pat. No. 2,946,763, describe a narrow range of melt-processible copolymers of tetrafluoroethylene and hexafluoropropylene having useful properties; and J. F. Harris, Jr., and D. I. McCane, U.S. Pat. No. 3,132,123, describe closely related melt processible copolymers of tetrafluoroethylene and perfluoroalkyl perfluorovinyl ethers.
These melt processible tetrafluoroethylene copolymers generally have melt viscosities in the range of 10.sup.3 to 10.sup.7 poise at their processing temperature. The melt extrusion coating of wire with some of these resins, using conventional single screw extruders, is described by D. I. McCane (Encyclopedia of Polymer Science and Technology, Vol. 13, pages 663-664, John Wiley & Sons, Inc., 1970).
The tetrafluoroethylene polymers useful in one of these two types of fabrication techniques cannot be used in the other type because any tetrafluoroethylene polymer useful for sintering fabrication technology for production of wire insulations must, of necessity, be a polymer having a molecular weight, crystal structure, and monomer content capable of fibrillation in paste extrusion to form the unsintered tape or unsintered wire covering and must then provide form stability and absence of cracking in the sintering step which follows. On the other hand, a tetrafluoroethylene copolymer useful in the melt processing technique must, of necessity, have a molecular weight and comonomer content to give the desired lower melt viscosity for melt processing, together with useful mechanical properties in the finished insulation. Thus, any fluorocarbon resin composition having the properties required for successful fabrication by paste extrusion and sintering is inherently unsuited for melt processing into wire insulation in the usual melt processing manner of operation. Its melt viscosity is much too high. Conversely, any fluorocarbon resin having the properties required for successful fabrication into wire insulation by melt processing in a conventional single screw extruder is inherently incapable of fibrillation as required for fabrication by paste extrusion and sintering.
3. Failure of Fluorocarbon Insulation at Low Voltage
Insulated electrical wire has commonly been made by both fabrication techniques described above using each respective type of fluorocarbon polymer. All such insulated wires exhibit insulation failure in wet locations when subjected to AC potentials in the range of about 600 volts. Specifically, they fail to pass an Insulation Resistance Test, described further below, in which they are subjected to 75.degree. C. water for 12 weeks with 600 volts AC stress applied across the insulation (i.e., between wire conductor and the surrounding water).
The prior art, e.g., U.S. Pat. No. 3,150,207 to W. L. Gore, teaches use of tetrafluoroethylene polymers to make insulated wire by the technique using a fibrillatable polymer and teaches that if a dielectric fluid, e.g., a silicone, is mixed with and incorporated in the polymer, the resulting insulation is resistant to corona as measured by subjecting the insulated wire to 3000-6000 volts in water containing a wetting agent. However, when such insulated wires are subjected to the Insulation Resistance Test discussed in the proceding paragraph, deterioration occurred under wet conditions, i.e., they undergo "AC wet service failure." Accordingly, fibrillatable tetrafluoroethylene polymer compositions having good corona resistance do not provide a means of achieving acceptable resistance to AC wet service failure; apparently the failure mechanism of wet service at low voltage (e.g., 600 volts) is different from that of corona attack at high voltage (e.g., 3000-6000 volts).